This invention relates to integrated circuits. More particularly, this invention relates to integrated circuits that provide high capacitance.
Very often, electrical circuits require a high capacitance ranging from a few tens of picofarads to a few thousand picofarads (i.e., nanofarads). Such high capacitance is used, for example, to compensate a servo loop or to delay signal timing. However, integrated circuits typically can provide capacitance only in the tens of picofarads because of the practicalities and economics of integrated circuit fabrication.
A common solution is to add an external or discrete capacitor to an integrated circuit requiring high capacitance. However, this requires a connection from the external capacitor to an integrated circuit package pin, which in many instances may not be available. Moreover, physical space for the addition of an external capacitor may also not be available depending on the component density and packaging of the system or device in which the integrated circuit is used.
As used herein, the term “integrated circuit” does not necessarily refer to a complete integrated circuit chip, but can instead refer to an integrated circuit portion of an integrated circuit chip. However, an integrated circuit does not refer to more than one integrated circuit chip.
Another known solution is to use area ratios of transistors on an integrated circuit to effectively “multiply” existing capacitance in the circuit to provide a desired high equivalent capacitance. However, capacitance multiplication is very sensitive to variations in transistor current gain, which in turn is sensitive to process variations and operating temperatures. Thus, known integrated multiplier circuits cannot be reliably fabricated with a specific effective capacitance, nor is an effective capacitance of known multiplier circuits likely to remain constant during subsequent circuit operation. Furthermore, only low multiplication factors (less than about 40) are possible because of transistor size limitations on integrated circuits.
In view of the foregoing, it would be desirable to be able to provide an integrated circuit having a high equivalent capacitance that does not require additional components external to the integrated circuit.
It would also be desirable to be able to provide an integrated circuit having a specified high equivalent capacitance that is substantially unaffected by transistor current gain variations.